The present invention relates generally to voltage biasing of bus lines, and more particularly to a method and apparatus for providing an adjustable biasing voltage for a bus line.
A computer bus is generally a set of parallel conductors connecting two or more electrical devices for the purpose of data transfer. One popular bus is the SCSI bus. The SCSI bus is an interface designed for computers and electronic instrumentation to allow communication of data over short distances. The SCSI bus is designed to connect independent devices such as disk drives, tape drives, file servers, video monitors, printers, scanners, and other computers. Several SCSI versions have been developed and standardized, with newer versions being developed to keep pace with changing computer speeds and requirements. One of the newer standards is low voltage differential (LVD) SCSI, an improved version of differential SCSI. Differential voltage signals (typically of 400 millivolts difference) are used in LVD SCSI, allowing for higher data transfer rates and longer distances than the older single-ended SCSI standard. In LVD SCSI, each differential data line of the SCSI bus is comprised of two separate bus lines. A binary zero is transmitted when a first bus line is more positive than a second bus line, and a binary one is transmitted when the second bus line is more positive than the first bus line.
SCSI devices are usually connected from one device to another in a daisy chain arrangement. This configuration gives rise to a transmission delay problem where the ends of the bus, not having the same characteristic impedance as the bus itself, will reflect a portion of the signal. This reflection will cause longer transmission times as the bus attempts to receive the data signal over the reflected signal. To lessen this reflection problem, the common practice is to terminate each end of the physical SCSI bus with a termination circuit or terminator that attempts to match the characteristic impedance of the SCSI bus. The terminators used in LVD SCSI commonly provide a fixed biasing voltage across the two bus lines to keep the receivers from being driven by noise in the absence of any differential signals.
Unfortunately, the prior art LVD terminators had several drawbacks due to the fixed biasing voltage. One such drawback was that variations in components used to implement the LVD terminators caused the LVD terminators to impress biasing voltages of slightly different magnitudes on the ends of the differential data line. This mismatch in biasing voltage generated loop currents through the differential data line during idle periods on the SCSI bus. These loop currents if severe enough caused differential voltages on the differential data line that receivers misinterpreted as data. Accordingly, prior art LVD terminators were manufactured Such that the LVD terminators had highly matched components that provided the ends of the differential data line with highly matched biasing voltages in order to reduce loop currents. This matching of components in the prior art LVD required costly manufacturing processes such as laser trimming of thin film resistors.
Therefore, what is needed is a cost effective method and apparatus for terminating a LVD SCSI data line that provides the differential data line with an adjustable biasing voltage in order to reduce loop currents on the differential data line.